A variety of aqueous hexavalent chromium containing passivating compositions that also contain phosphate and one of the fluorometallate ions noted above are known in the art, for example as taught in U.S. Pat. Nos. 5,807,442 of Sep. 15, 1998 to Goodreau; U.S. Pat. No. 5,091,023 of Feb. 25, 1992 to Saeki et al.; U.S. Pat. No. 4,749,418 of Jun. 7, 1988 to Saeki et al.; U.S. Pat. No. 4,668,305 of May 26, 1987 to Dollman et al.; all of which, to the extent not inconsistent with any explicit statement herein, are hereby incorporated herein by reference.
With increasing recognition of the environmental and safety impact of hexavalent chromium, attempts have been made to replace all or some of the hexavalent chromium in passivates with trivalent chromium, for example as taught in U.S. Patent Application Publication No. 2004/0173289. However, doing so has resulted in unforeseen challenges. Some conventional trivalent chromium passivating working baths lose stability after ageing due in part to dissolution of metal from the substrates, particularly substrates having zinciferous surfaces, into the bath. Thus, zinc build-up in working baths of trivalent chromium passivates is a significant problem industrially where for example rapid metal coating processes can run in excess of 100 sq ft per minute through baths.
Prior art Cr (VI) passivating compositions used the oxidizing nature of hexavalent chromium to inhibit dissolution of metal from the substrate into the bath, which gave the Cr(VI) working baths adequate stability. In replacing Cr(VI) with Cr(III) in passivating baths, oxidative inhibition of metal dissolution from the substrates was lost and bath instability resulted. Conventional thinking has taught that other oxidizers, such as nitrates and peroxides, replacing Cr (VI) should be added to the Cr (III) passivating compositions so that, when the compositions were made into working baths, the oxidizers would inhibit dissolution of the metal substrate into the bath. This prior art approach had some success, but caused other problems and limitations on additions to the coating compositions. For example, the presence of these replacement oxidizers resulted in production of toxic gases, such as NO and CO2, by reaction of the oxidizers with any organic material, in particular residual organic material used to reduce the Cr(VI) to Cr(III) in the passivating composition. The presence of oxidizers also limited the use of other organic additives that might be beneficial to the extent that the organic additive could be predicted to react with an oxidizer. Thus, there is a need for a means of reducing build-up of Zn in Cr (III) working baths in the absence of Cr (VI), and in the absence of other oxidizers in the bath which react to produce noxious gasses.
Another drawback of conventional Cr (VI)-free, trivalent chromium-containing coatings is that they provide reduced corrosion resistance of the coated metal substrate as compared to similar substrates passivated using Cr (VI)-containing chromium compositions. Conventional Cr (VI)-free, trivalent chromium-containing passivate compositions also require higher amounts of phosphate to stabilize the Cr (III) in the bath, but the presence of the excess phosphate also has drawbacks including reducing corrosion resistance (for example in the neutral salt spray test) and increased staining of coated substrates. Thus, there is a need, particularly in passivating zinciferous surfaces, for a composition and process that provides improved product stability and better corrosion and stain resistance of coated substrates.